File templates dialog: Fix crash in overrides page with unknown base class

Summary:
Having entered a base class which is unknown to the DUChain,
on preparing the page with overrides KDevelop crashes.

This is fixed by catching the case that there is no
context available for a base class.

Reviewers: KDevelop, mwolff

Reviewed By: KDevelop, mwolff

Subscribers: mwolff, kdevelop-devel

Differential Revision: https://phabricator.kde.org/D4527